1. Field of the Invention
The present invention is generally in the field of electrical waveform generators, and, more particularly, is in the field of sawtooth waveform generators having an operational amplifier with a switched capacitor between the output and the inverting input.
2. Description of the Related Art
A sawtooth waveform is a periodic waveform that has a ramp portion that changes linearly from a starting value to an ending value during a predetermined time interval and then resets rapidly to the starting value at the beginning of the next time interval. Generally, the ramp portion increases from a low starting value to a high ending value during each time interval; however, for some applications, the ramp portion may decrease during the time interval.
Sawtooth waveforms are commonly used in pulse width modulation (PWM) systems, in frequency synthesizers and other systems requiring a signal that changes linearly over a predetermined time interval.
Historically, a sawtooth waveform was generated by charging a capacitor using a constant current source and then discharging the capacitor at the end of each time interval. However, discharging the capacitor at the end of each interval required a finite time and caused the reset transition of the sawtooth waveform to have a slope determined by the impedance in the discharge circuit. Although the time required to reset the waveform may be insignificant for a low frequency sawtooth waveform, the time required to reset the waveform is a significant portion of the overall period for a higher frequency sawtooth waveform, thus causing the sawtooth waveform to have the appearance of a triangular waveform.
More recently, sawtooth waveform generators have not discharged the capacitor to reset the sawtooth waveform to the starting value. Rather, when the ramp voltage on the output of the operational amplifier reaches a maximum value at the end of each time interval, the connections to the capacitor between the output of the operational amplifier and the inverting input of the operational amplifier are rapidly switched while maintaining the charge on the capacitor. The switching of the connections to the capacitor causes the operational amplifier to rapidly switch the voltage on the output of the operational amplifier to maintain the voltage on the inverting input substantially equal to a reference voltage on the non-inverting input. The capacitor is then linearly charged in the opposite direction as the operational amplifier generates an identical ramp voltage during a second time interval. Since the capacitor is not rapidly discharged between the time intervals, the reset transition occurs as quickly as the connections to the capacitor are reversed and the operational amplifier responds to the reversed voltage on the inverting input.
Known sawtooth waveform generators using the above-described switched capacitor rely on flip-flops to switch the connections to the capacitor during alternating time intervals; however, the circuitry controlling the flip-flops is subject to entering an unknown state or failing to switch at the end of an interval such that the sawtooth waveform generator may halt.